This disclosure relates to polycarbonate compositions, and in particular to static resistant polycarbonate compositions, methods of manufacture, and uses thereof.
Polycarbonate is a high-performance plastic with good impact strength (ductility). Polycarbonates, especially aromatic polycarbonates, are routinely used in structural applications, for example, in the form of headlight lens for automotive applications. They are also the common materials for the fabrication of optical data recording media, including optical disks as exemplified by compact audio disks and CD-ROM disks used in computers.
Polycarbonates can accumulate a surface charge of static electricity, which can reduce the desirable qualities of the polycarbonate composition in such applications where it can attract dust and other materials, by compromising the transparency and other surface finish properties of an article prepared from the polycarbonate.
It is known in the art to add different types of anti-static agents to the polycarbonate to reduce dust attraction. U.S. Pat. No. 6,194,497 discloses an anti-static thermoplastic resin composition of 90 to 99.95 weight percent (wt %) of a thermoplastic resin and correspondingly 0.05 to 10 wt % of a halogenated carbon sulfonic acid salt of a polysubstituted phosphonium compound such as a fluorinated phosphonium sulfonate. Antistatic agents, such as tetrabutyl phosphonium perfluorobutanesulfonate (sometimes referred to as “FC-1”) have been added to polycarbonate (European Patent No. 897950A3), where a molded part formed from the combination has been disclosed to dissipate an electrostatic surface charge. Prevention of the build up of electrostatic surface charge is also disclosed to have been accomplished by including antistatic end groups in a polymer as in, for example, U.S. Pat. No. 6,140,457 which discloses a di-t-alkylphenyl end group as a preferred static reducing end group. Further, dust patterns are disclosed to have been reduced in polycarbonates by addition of a polyalkylene ether (U.S. Pat. No. 6,949,596).
Polycarbonate is well known for its electrical insulating properties. Typical surface resistivity values are in the range of 1018 ohm/square. As a result of extrusion and injection molding the surface of the polycarbonate part can have a electrostatic surface charge that is either positive or negative in polarity. In both cases parts formed from the polycarbonates will attract dust from the environment. When the surface is charged negatively the dust patterns appear in a fractalized pattern that is shaped like fern marks. A positive electrostatic surface charge will result in circular halo-like patterns. Both such kinds of pattern are considered aesthetic defects, especially in lighting applications.
A similar problem occurs when complex articles, such as automotive headlamp lenses, are molded from the polycarbonate composition. When exposed to the environment, dust is attracted onto the polycarbonate article and patterns form that may be circular or irregular in nature and that therefore are aesthetically unattractive. This problem may occur even when antistatic agents are present in the composition. However, addition of an antistatic agent in amounts sufficient to reduce the incidence of problem can also degrade the mechanical and optical properties of the polycarbonate.
There accordingly remains a need in the art for polycarbonates having high resistance to the development of charges of static electricity and methods of making the same.